(1) Field of the Invention
The present invention relates to an adsorptive separation method for adsorbing and separating an intended component from a mixture comprising at least two substances at a purity higher than the aimed purity in an economically advantageous manner, which can easily cope with scale-up of the apparatus.
More particularly, the present invention relates to an adsorptive separation method in which a mixture comprising a plurality of substances and a desorbent are alternately supplied to a column packed with an adsorbent and while an adsorption band of the mixture of said substances is being formed and moved, the intended component is separated from the eluate portion at a purity higher than the aimed purity economically advantageously and which can easily cope with scale-up of the apparatus.
(2) Description of the Prior Art
As a method for separating and obtaining an intended component at a high purity from a mixture comprising a plurality of substances, there are known various methods such as the distillation separation method using a rectifying column, the low-temperature processing crystallization method and the solvent extraction method, and these methods are carried out on an industrial scale according to the properties of substances to be separated. Furthermore, industrialization of the adsorptive separation method using an adsorbent having a selective adsorbing capacity and a desorbent has recently been increased. The development of this adsorptive separation method is due to the fact that various adsorbents having a high selective adsorbing capacity have been developed and various adsorptive separation conditions for these adsorbents combined with desorbents have been created and also to the fact that the conditions for separating substances that cannot be separated according to the conventional methods have been revealed. There also is known a method in which components having a low relative volatility are rectified at a large stage number and a high reflux ratio. This method, however, is defective in that the consumption of steam is great and cannot cope with the recent increase of energy expenses As a method for solving this problem, the adsorptive separation method utilizing the selective adsorbing capacity is promising.
As the solid adsorbents now used on an industrial scale, there can be mentioned not only synthetic zeolites but also active carbon, silica gel, active alumina, ion exchange resins, polystyrene gel, clay, diatomaceous earth and porous glass. These solid adsorbents have inherent characteristics, respectively, and appropriate adsorbents should be selected according to the object of the separation operation while taking the adsorbing characteristics of the adsorbents, the properties of the starting mixtures and the lives and costs of the adsorbents into consideration. Recently, various designs of the micro- and macro-structures and various kinds of post-treatment (such as metal ion exchange and acid treatment) have been developed so as to improve these solid adsorbents for obtaining high selective adsorbing characteristics, and significant progress has been made in the field of solid adsorbents.
As the industrial process using a zeolite as an adsorbent, there are known the separation of n-parafins, the separation of n-olefins, the separation of butene-1, the separation of xylene, cymene and diethylbenzene isomers, the separation of cyclohexane from cyclohexene, the separation of -pinene and -pinene and the separation of fructose from glucose.
It is well-known that a so-called high performance liquid chromatography using high-capacity porous particles having a particle size of about 10 .mu.m, such as Zipak.RTM., Cerasil.RTM., Perasil.RTM., Sil.RTM. and Zorbax.RTM., has recently been developed widely as a means effective for the separation of organic substances.
As another method for separating an intended component from a mixture comprising a plurality of substances, there is known a method using an ion exchange resin. For example, this method is utilized for the separation of fructose from glucose, the separation of rare earth metal ions, the separation of organic and inorganic acids and the separation of amino acids.
These methods for the adsorption and separation of mixtures comprising a plurality of substances by using solid adsorbents and liquid desorbents are included in the category of the technique called "liquid-solid column chromatography" in a broad sense. More specifically, when a liquid of a mixture is passed through a column (tower) packed with a solid adsorbent, the respective components of the mixture are distributed into the adsorbent side (stationary phase) and the moving liquid side (mobile phase) at different ratios according to the solid-liquid equilibrium relationship, that is, the difference in the adsorptive force, and therefore, the difference in the moving speed is brought about among the components of the mixture and separation is thus effected. The respective components thus separated are allowed to flow out from the downstream side of the column, and the effluent is continuously or intermittently analyzed to determine the concentrations of the components in the effluent and the fraction of the effluent containing the intended component is recovered.
In the case of the conventional adsorptive the separation technique according to the liquid chromatography, separation of the desired component from a mixture of inorganic or organic substances is ordinarily accomplished according to the following procedures:
(1) First, a mixture containing a substance to be separated and recovered is supplied as the starting mixture to a column (adsorption column) packed with an adsorbent (at this step, the mixture forms an adsorption band in the upstream portion of the column).
(2) Then, a desorbent is supplied to move the adsorption band downward (during this moving step, the respective components are gradually separated according to the difference in the adsorbing forces of the adsorbent for the respective components).
(3) The desorbent is further supplied to the column and the separated components are eluted from the column in sequence.
(4) The eluate fraction containing the intended component having the aimed purity is recovered.
(5) The desorbent in the eluate fraction is removed and the intended component is recovered.
In the foregoing manner, the mixture comprising a plurality of substances and the desorbent are alternately supplied into the column packed with the adsorbent, and the above procedures (1) through (5) are repeated, whereby the intended component is separated and recovered at the aimed purity. This method is widely adopted on an industrial scale.
This separation method, however, involves problems to be solved, which will now be described in detail with reference to an embodiment in which a mixture comprising a substance X for which the adsorbent has a weak adsorbing force and a substance Y for which the adsorbent has a strong adsorbing force is supplied in a column and the substance Y is separated as the intended component. Of course, this embodiment is given for facilitating the understanding.
The graph on which the change in the concentration of the component in the effluent at the outlet of the column according to the elution time (or the amount of the effluent) is plotted is called a "chromatogram". In the case where the adsorbing forces of the adsorbent to the substances X and Y do not greatly differ from each other or where the developing distance is short even if there is a great difference between the above adsorbing forces, the components X and Y are not completely separated (by the term "complete separation" is meant a state where both the components X and Y are not co-present), and the chromatogram curves of the components X and Y overlap each other and there is obtained an effluent in which both the components X and Y are co-present. For example, a chromatogram as typically shown in FIG. 1 is obtained. A long developing distance is required for attaining complete separation of these components X and Y. Accordingly, the scale of the adsorption column inevitably must be increased and therefore the method is not suitable for the practical operation. Furthermore, as the adsorption band moves in the column, the adsorption band is expanded and the concentration of the intended component is gradually reduced, with the result that the productivity of the adsorption column is reduced and the load on the separation of the intended component from the desorbent by distillation, extraction and recrystallization is increased. Therefore, complete separation is not always advantageous from an industrial viewpoint. In the actual operation, it often happens that the incorporation of the non-intended component (component X) into the intended component (component Y) is not practically disadvantageous if the concentration of the non-intended component is lower than the allowable concentration. In this case, complete separation of the components X and Y is not absolutely necessary. Accordingly, the method is advantageously carried out on an industrial scale only when for the economical reasons, for example, in view of the manufacturing costs and the cost of construction of installations such as columns, it is permissible to separate and recover the intended component without complete separation of the components X and Y.
In the case where a chromatogram as shown in FIG. 1 is obtained and the component Y is recovered at a purity of, for example, 90% (the component X is included as an impurity in an amount of 1/9 of the amount of the component Y), at the time (t.sub.1) on the chromatogram, valves outside the column are changed over or another necessary operation is performed so that the effluent after the time (t.sub.1), that is, the effluent in which the entire purity of the component Y is higher than 90%, is collected, and if the desorbent is removed from the effluent, the component Y having a purity higher than 90% can be obtained at a maximum recovery ratio. On the other hand, the effluent before the elution time (t.sub.1) is discarded (the component Y contained in the hatched portion in FIG. 1 is lost), or this effluent is utilized again. For example, in the case of the separation of p-xylene from a mixture of xylene isomers, the effluent containing a smaller amount of p-xylene is distilled and then fed to the isomerization step to form p-xylene according to the chemical equilibrium, and the so-treated effluent is fed as the starting material to the chromatographical process again.
The problems involved in the above-mentioned conventional adsorptive separation method are summarized in the following two points:
(1) It is impossible to increase the recovery ratio without any economical sacrifice, and it is practically impossible to obtain a recovery ratio of 100%. A method for obtaining such a high recovery ratio is not optimal and the application range of this method is drastically limited, and in this case, the recovery ratio or purity is very low.
(2) The chromatogram is greatly changed (disturbed) due to scale-up of the apparatus, and a large-diameter absorption column cannot advantageously be utilized.
These two points will now be described in detail.
At first, the point (1) is discussed. As pointed out hereinbefore, when there is not a great difference between the adsorbabilities of the components X and Y, or when the developing distance is short even if there is a great difference between the adsorbabilities of the components X and Y, the components X and Y are not completely separated (by the term "complete separation" is meant the state where both the components X and Y are not co-present), and the chromatogram curves of the components X and Y overlap each other and there is obtained an effluent in which both the components X and Y are co-present. For example, a chromatogram as typically shown in FIG. 1 is obtained. A long developing distance is required for attaining complete separation of these components X and Y. Accordingly, the scale of the adsorption column is inevitably increased and the method is not suitable for the practical operation. Furthermore, as the adsorption band moves in the column, the adsorption band is expanded and the concentration of the intended component is gradually reduced, with the result that the productivity of the adsorption column is reduced and the load on the separation of the intended component from the desorbent by distillation, extraction and recrystallization is increased. Therefore, complete separation is not always advantageous from an industrial viewpoint. Accordingly, the adsorptive separation method is advantageously performed on an industrial scale only when for the economical reasons, for example, in view of the manufacturing costs and the cost of construction of installations such as column, it is permissible to separate and recover the intended component without complete separation of the components X and Y. In short, the application range of the method is limited.
In the conventional method, the following problem should be solved so as to obtain the intended component stably. As pointed out hereinbefore, a maximum recovery ratio (highest yield) can be obtained when collection of the effluent is started at the time (t.sub.1) in FIG. 1 where the highest allowable concentration of the component X appears. If collection of the effluent is started at a point after the time (t.sub.1), the recovery ratio is reduced. On the other hand, if collection of the effluent is started at a point before the time (t.sub.1), the purity of the product Y becomes lower than the aimed purity and the recovered product cannot be used.
Namely, in the conventional method, if cutting is not effected precisely at the intended cutting time (t.sub.1), the recovery ratio or the purity is reduced. Moreover, it is very difficult to determine the cutting point (t.sub.1) before elution of the total amount of the component Y is completed. Accordingly, in the case where the conventional method is carried out on an industrial scale, predetermined amounts of the mixture and desorbent are alternately supplied precisely at constant flow rates into the column, and cutting is performed at the time (t.sub.1) determined as the cycle of a certain time. However constant flow rates and feed amounts may be maintained, disturbances of chromatograms are considerably large when the method is carried out on an industrial scale. Because of not only these disturbances but also variations of the mixing ratio of the components X and Y and their amounts, it is very difficult to effect cutting precisely at the predetermined point even if highly advanced recent control units, high-precision valves, high-precision metering pumps and analyzing devices are thoroughly utilized.
Accordingly, it is necessary that the cutting point should be set so that a certain deviation in the cutting point has no influence on the purity of the product. Namely, a point on the right side of the point (t.sub.1) in FIG. 1 should be set as the cutting point. However, from FIG. 1, it will readily be understood that this margin for the deviation will result in substantial reduction in the recovery ratio.
Ordinarily, if the cutting time is deviated by 1 second from the point (t.sub.1), the purity or the recovery ratio is reduced by 0.1 to 10%, though this value differs to some extent according to the substance to be separated or the separation system.
In order to effect adsorptive separation economically advantageously, it is necessary to solve the foregoing problems. Needless to say, these problems should also be solved when a mixture comprising at least three components is treated.
The point (2) will now be discussed.
When industrialization of an adsorption column is intended, usually, an apparatus of a bench scale is first tested and the scale is then increased to a pilot apparatus and further to a commercial apparatus. From the principle of the adsorptive separation, scale-up of the adsorption column is ordinarily accomplished by increasing the diameter while the developing distance, that is, the length of the column, is kept constant. This increase of the column diameter results in the non-uniformity of the packing structure in the radial direction of the column or the non-uniformity of distribution or confluence of the fluid, and the degree of disturbance of the chromatogram at the outlet of the column is larger than in the small-diameter column.
In the case of a column customarily adopted for ordinary adsorptive separation where the column length is much larger than the column diameter, the disturbance of the chromatogram is often more influenced by the non-uniformity of the packing structure in the radial direction than by the non-uniformity of the distribution or confluence of the fluid. The change of the chromatogram due to scale-up of the column diameter is diagrammatically illustrated in FIG. 2.
In FIG. 2, solid lines indicate typical chromatograms obtained when a mixture of two components X and Y is subjected to the adsorptive separation in a small-diameter column, and broken lines indicate chromatograms obtained when in a large-diameter column packed with the same adsorbent at substantially the same pack density, development is carried out under the same adsorption conditions by using the same desorbent. Since the adsorption conditions are the same, the degree of separation of the components represented by the distance between the peaks does not greatly differ between the two cases, but in the case of the large-diameter column, since non-uniform streams of the fluid are gathered, the respective peaks become broad and the overlapping region of the peaks is increased, and the cutting position for obtaining the intended component Y at a predetermined purity is retreated to the point (t'.sub.1) from the point (t.sub.1). Accordingly, the recovery amount of the component Y in case of the large-diameter column, which corresponds to the area defined by A', B' and D', is much smaller than the recovery amount in the small-diameter column, which corresponds to the area defined by A, B, C and D. Therefore, scale-up results in economical disadvantages, and in an extreme case, scale-up becomes practically impossible.
For example, when p-xylene is adsorbed and separated from a mixture of xylene isomers by using a zeolite as the adsorbent, if the column diameter is smaller than 20 mm, a disturbance in the chromatogram due to channeling has no substantial influence on the recovery ratio, though the value differs to some extent according to the intended purity or kind of the substance to be separated. However, if the column diameter is larger than 20 mm, the recovery ratio is reduced according to the packing method and in an extreme case, the p-xylene recovery ratio is lower than 1/3 of the recovery ratio attained when the column diameter is 20 mm or less. This reduction in the recovery ratio is especially significant when the column diameter exceeds 200 mm, and it sometimes happens that recovery of p-xylene at an aimed purity is impossible because of long tailing of the components other than p-xylene. This problem is serious when the column diameter is 5,000 to 6,000 mm as in the case of a commercial column. In this case, the separation cost is increased owing to a reduction in the recovery ratio, and furthermore, it becomes substantially impossible to practically carry out the separation process on an industrial scale.
Ordinarily, for minimizing this influence by scale-up, contrivances have been made only on the method for packing the adsorbent so as to uniformalize the packing structure of the adsorbent. Of course, it is necessary to uniformalize the packing structure as much as possible, but in the case of a column having a diameter larger than 200 mm, this uniformalization is limited and the packing structure is inferior in the packing uniformity more or less to the packing structure attainable in a column having a diameter smaller than 20 mm. Furthermore, as taught in many patent specifications, many labors are required for attaining the uniform packing according to the known packing methods.
Moreover, even if the packing structure is uniformalized to such an extent that no large and broad flow rate distribution is produced, in the case where the mass transfer rate to the point of adsorption of the adsorbent from the fluid is very slow, that is, in the case where the substance having a higher adsorbability is pushed out by the substance having a lower adsorbability, the channeling or disturbance is increased even by a slight flow rate distribution in the radial direction of the column. As is seen from the foregoing description, the problem of channeling in the large-diameter column is related to various factors, and this problem cannot be solved only by attainment of a uniform packing structure. However, it is necessary to establish a flow method not causing channeling and a separation process allowing a certain channeling.
As a method for moderating the foregoing defects of the conventional adsorptive separation method, there can be mentioned a technique using a simultaneous moving bed in the method for separating hydrocarbons by using zeolites (see Japanese patent publications No. 15681/67, No. 17643/68 and No. 24243/71).
According to this method, since the adsorbent and developing agent are supplied in a counter-current manner, a constant chromatographical curve can be obtained, and therefore, the component Y can be withdrawn continuously from the region where the component X is not incorporated. In other words, the problem (1) of the conventional method is solved by this method and the adsorptive separation can be performed stably while maintaining the recovery ratio substantially at 100%. However, this method is defective in that the apparatus and system to be used become very complicated and the construction costs are much greater than in the conventional method. Moreover, the problem (2) cannot be solved by this method at all. More specifically, the disturbance of the chromatogram due to channeling caused by scale-up of the column diameter cannot be prevented at all by this method. Accordingly, although a certain recovery ratio can be attained according to this method, the obtained chromatogram is inevitably considerably flat. Therefore, the amount used of the desorbent per unit amount of the intended component is drastically increased and the cost of the energy necessary for removing the desorbent from the intended component is increased. Furthermore, since the width of the chromatographical band is increased, the utilization efficiency of the column is reduced.
We made researches with a view to eliminating the foregoing defects of the known improved technique while retaining advantages, such as simplicity and expediency, of the conventional adsorptive separation method, and as a result, we have now completed the present invention described below.
It is therefore a primary object of the present invention to provide an improvement in the method comprising alternately supplying a starting mixture comprising a plurality of substances and a desorbent into a column packed with an adsorbent and recovering the desired component at a purity higher than the aimed purity from the eluate portion while forming and moving an adsorption band of the starting mixture, in which (1) the recovery ratio of the intended component is increased and the separation is accomplished more economically advantageously than in the conventional method, and (2) the disturbance of a chromatogram caused by scale-up of the column diameter is coped with by (A) providing a flow method capable of substantially preventing occurrence of the disturbance or (B) providing a system allowing a certain disturbance so that the recovery ratio is not substantially influenced by such disturbance.